1. Field of the Invention
The present invention relates to a profilometer and method for measuring, with high precision, the surface profile of an ultra-small area; for example, the surface profile of a pickup lens of an optical disk, that of a small-diameter lens to be used in optical communication, such as a fiber condenser lens, or that of a mold for the lens.
2. Description of the Related Art
Japanese Patent Application Laid-Open Nos. H04-299206, H10-170243, and Japanese Patent No. 2748702 describe an ultrahigh precision three-dimensional profilometer capable of measuring the surface profile of an aspheric lens or that of a mold for the lens. In the three-dimensional surface profiling method, there are some kinds of method; for example, a method to directly contact to an object by a probe, a measurement method using an optical probe and utilizing a behavior of an optical interference, or such. The Japanese Patent No. 2748702 discloses an error correction method by using a reference spherical reference ball in a profilometer including an optical probe. FIG. 1 is a perspective view showing an example construction of the profilometer.
The profilometer is constructed such that a tip end of a stylus 5 attached to a moving element 3 is caused to follow a surface of measurement 2a (hereinafter called a xe2x80x9cmeasurement surfacexe2x80x9d) of an object of measurement 2 (simply called an xe2x80x9cobjectxe2x80x9d), such as a lens, placed on a stone surface plate 1, thereby measuring the surface profile of the object 2. More specifically, an X reference mirror 6, a Y reference mirror 7, and a Z reference mirror 8, which are intended for measuring the positional coordinates of the probe 5 by way of a support section, are placed on the stone surface plate 1 on which the object 2 is to be placed. The moving element 3 having the probe 5 attached thereto is equipped with an X stage 9 and a Y stage 10. The moving element 3 and the probe 5 can be scanned in both the X-axis and Y-axis directions by means of following the surface profile of the measurement surface 2a of the object 2. The moving element 3 is equipped with a laser length-measuring optical system 4. By means of a known light interference method, the profilometer measures the X coordinate of the probe 5 with reference to the X reference mirror 6; the Y coordinate of the probe 5 with reference to the Y reference mirror 7; and the Z coordinate of the probe 5 with reference to the Z reference mirror 8.
Measurement procedures for use in such a profilometer will now be described. First, design information, such as an equation representing the design profile of the measurement surface 2a of the object 2, is input to the profilometer before measurement. Next, the probe 5 is caused to follow the measurement surface 2a of the object 2 at a constant measurement pressure. Centering of the object 2 is effected, by means of causing the probe 5 to perform axial scanning in the X and Y directions. Details on the centering operation are described in Japanese Patent Application Laid-Open No. 254307/1990. Subsequently, the probe 5 actually scans the measurement surface 2a of the object 2 in the X and Y directions, thereby measuring the profile of the surface.
FIG. 10 shows a view of how a stylus 31 provided at the extremity of the probe 5 of the profilometer follows the measurement surface 2a of the object 2, when enlarged in the Z and X coordinates. Three-dimensional coordinates detected by the stylus 31 correspond to coordinates (X0, Y0, Z0) of the tip end T of the stylus 31 shown in FIG. 10. However, as illustrated, a tip end section 32 of the stylus 31 has a curvature radius R. When the tip end section 32 is following the surface profile of the object 2, a measurement error arises between three-dimensional coordinates (Xi, Yi, Zi) of an actual point of measurement P and the coordinates (X0, Y0, Z0) of the tip end T of the stylus 31 obtained as a result of scanning operation of the probe 5.
If the inclination angle xcex8 of the measurement surface 2a at the actual position of point P of measurement is known, coordinates (Xi, Yi, Zi) of the actual point P of measurement can be computed from the coordinates (X0, Y0, Z0) of the tip end T of the stylus 31. A measurement error derived from the curvature radius R of the tip end section 32 of the stylus 31 can be corrected by means of subtracting or adding the position of actual point P of measurement relative to the tip end T of the stylus 31 (i.e., a relative distance between two coordinates).
In connection with the Z-X coordinates, provided that coordinates of the tip end T of the stylus 31 belonging to the probe 5 assume (X0, Y0, Z0); that coordinates of an actual point P of measurement assume (Xi, Yi, Zi); and that the angle of inclination of the measurement surface 2a in the X direction assumes xcex8x, then (Xi, Yi, Zi)=(X0xe2x88x92Rxc2x7sin xcex8x, Y0, Z0+R, (1xe2x88x92cos xcex8x)) (where coordinate components Yi, Y0 in the Y direction in the Z-X coordinates are indefinite). Similarly, if the inclination angle xcex8 of the measurement surface 2a in the Y direction at the actual position of point P of measurement is known, the same correction can be made to the Z-Y coordinates. Correction of such a measurement error (i.e., an R error of the extremity of the probe) derived from the curvature radius R of the tip end section of the stylus belonging to the probe will hereinafter be called probe R correction. The inclination of angle xcex8 obtained at this time can be computed from previously-acquired or subsequently-acquired measurement data. Alternatively, the inclination of angle xcex8 can also be determined by means of the coordinates of the tip end T of the stylus 31 and the design equation of the object 2.
Surface-profiled data pertaining to the object 2 detected by the stylus 21 include a placement error which has arisen at the time of placing of the object 2 (i.e., an alignment error). When occurrence of an error between the surface-profiled data and the input design formula has been determined, the coordinate system is transformed by means of three-dimensionally rotating and translating the data that have been subjected to probe R correction, thereby optimally superimposing the data onto the design equation. As a result, an alignment error is corrected. Subsequently, the probe R correction and the transformation of a coordinate system will be hereinafter collectively referred to as alignment processing.
After alignment processing, there is determined a profile error (deviation) in the Z direction between the input design equation and the measurement data pertaining to the object 2, and deviation data are output. When a large profile error exists between the design equation and the actual object, the deviation data are fed back to a processing machine. Processing is repeated until the actual profile of the object 2 falls within a range of desired precision as compared with the design equation (e.g., a profile error falls within a range of xc2x10.1 xcexcm in the case of an aspheric pickup lens for use with an optical disk), thereby manufacturing an aspheric lens or a mold thereof; that is, the object 2, with high precision.
In the case of the ultrahigh precision three-dimensional profilometer capable of effecting measurement with high precision on the order of 50 nm or less, the tip end section 32 of the stylus 31 attached to the probe 5 which follows the surface profile of the object 2 is required to assume a high sphericity of 0.02 to 0.03 xcexcm or less and excellent durability against repeated measurement. For this reason, there has widely been employed a ruby ball with an outer diameter of 1 mm or thereabouts which can achieve a high degree of sphericity through mechanical polishing and has superior machinability and hardness characteristics.
In recent years, in the field of optical communication, an optical fiber condenser lens used in a photoelectric transducing device has a small lens diameter of about 2 mm or less and a surface inclination angle of 40xc2x0 or more. In relation to such an optical fiber condenser lens, a lens-barrel-integrated aspheric surface glass lens into which a lens barrel and a lens are bonded into a single piece at the time of pressing operation has been employed for facilitating handling.
FIG. 11 illustrates an example of contact status of the stylus 31 when an optical fiber condenser lens 41 is measured through use of the stylus 31 having the probe 5 attached thereto. The tip end section 32 of the stylus 31 is a ruby ball of about 1 mm. Hence, when the profile of the optical fiber condenser lens 41 having a lens diameter of 2 mm or less is measured, interference arises between the stylus 31 or the ruby ball serving as the tip end 32 thereof, and the lens barrel 42 of the optical fiber condenser lens 41, at the end of an effective diameter W of the lens. There arises a problem of difficulty in measuring the entirety of the effective radius W of the optical fiber condenser lens 41.
In order to measure the entirety of the effective radius W of the optical fiber condenser lens 41, the curvature radius of the tip end section 32 of the stylus 31 must be reduced to several micrometers. At this time, for instance, there is conceived a case where diamond is attached to the stylus 31 in place of the ruby ball of 1 mm and the curvature radius of the tip end of the stylus is made small, to several micrometers, through polishing. In this case, in contrast with a ruby ball, diamond cannot be produced through rolling polishing. Hence, an expert engineer must manually polish diamond. Further, a diamond crystal has orientation, which deteriorates machinability. In consideration of these factors, difficulty is encountered in achieving a desired degree of sphericity. For instance, when the tip end section 32 assumes an open angle of 120xc2x0 or less, a sphericity of 0.1 xcexcm or thereabouts is obtained or, in a worst case, a sphericity of 0.2 xcexcm or more. Thus, the sphericity of diamond becomes several times or more worse than that of a related-art ruby ball.
When the surface profile of the object 2 is measured through use of the stylus 31 made of diamond of poor sphericity, the low sphericity directly appears in the measurement data as an error in the profile of the object 2, because an error in the profile of the tip end section 32 of the stylus 31 is not corrected through the probe R correction, thereby hindering high-precision measurement.
A higher numerical aperture has recently been pursued even for a small-diameter pickup lens for use with an optical disk. Some of the lenses have an inclination angle of 40xc2x0 or more. If an attempt is made to measure the surface profile of the lens or the mold therefore by a conventional profilometer with a probe directly attaching to the object, interference arises between the tip end of the stylus and an edge surface. Moreover, if do the same by using a conventional profilometer including an optical probe, the reflected light from the object cannot be acquired enough to profile. Hence, it is hard for the conventional profilometeres to measure the entirety of the effective diameter of the lens and that of the mold. Therefore, a profiling method and apparatus to measure forces between atoms of a probe and an object is suggested. Additionally, if a probe, whose tip portion has smaller diameter than the conventional ruby ball, is applied, it may be possible to profile the object that has an inclination angle of 40xc2x0 or more with high precision. However, as mentioned above, the diamond ball, which has smaller diameter than the ruby ball, has poor sphericity and the poor sphericity thereof causes the low-precision measurement.
The present invention has been conceived in view of the foregoing circumstances and aims at providing a profilometer and method which enable high-precision measurement without involvement of interference between the tip end of a stylus and an edge surface of a small-diameter lens or a lens barrel even at time of surface profiling of a small-diameter lens, such as an optical fiber condenser lens used in optical communication or a pickup lens for use with an optical disk or surface profiling of a mold of the lens, as well as a method of manufacturing an object of measurement.
First, the invention provides a profilometer which measures the surface profile of an object of measurement by means of causing an extremity of a probe to follow a measurement surface of the object, comprising:
a probe having a curvature radius of 1 mm or less provided at the extremity of the probe;
curvature correction means for correcting the measurement data pertaining to the object in connection with a positional error stemming from the curvature radius of the probe; and
stylus profile correction means which corrects an error in the profile of the probe through use of profile error data pertaining to the probe determined through measurement of an article of reference shape which is to act as a reference for calibration.
Second, the article of reference shape employs a spherical reference ball.
Third, the probe is machined so as to have a tip-end open angle of 55xc2x0 or less and such that a tip end has a curvature radius on the order of micrometers.
Fourth, the tip end of the probe is made of diamond.
Fifth, the invention provides a surface profiling method for measuring the surface profile of an object of measurement by means of causing a probe to follow a measurement surface of the object, wherein a probe having a curvature radius of 1 mm or less is provided at the extremity of the probe, the method comprising:
a probe shape computation step of determining a profile error stemming from the curvature radius of the probe, by means of measuring a reference ball to be used as a reference for calibration; and
a probe shape correction step of correcting measurement data pertaining to the object through use of the stylus profile error data obtained through measurement of the reference ball.
Sixth, the probe shape correction step includes:
a curvature correction step of determining a contact position between the object and the probe and correcting the measurement data pertaining to the object in connection with a positional error stemming from a curvature radius of the probe on the basis of an angle of inclination of the measurement surface in the contact position; and
a profile error correction step of extracting the amount of profile error in the contact position from the stylus profile error data and correcting the profile error stemming from the curvature radius of the probe by means of addition or subtraction of the amount of profile error.
Seventh, in the profile error correction step, a position on the reference ball corresponding to the contact position of the probe at the time of measurement of the reference ball is specified through use of the angle of inclination of the measurement surface in the contact position between the probe and the object, and a profile error in a specified position on the reference ball at the time of measurement of the reference ball is extracted and corrected as the amount of profile error in the contact position of the probe.
Eighth, in the profile error correction step, the amount of profile error in the contact position of the probe is determined through interpolation, on the basis of deviation data which have been discretely acquired as the stylus profile error data at the time of measurement of the reference ball and pertain to a design equation.
Ninth, in the profile error correction step, a spline curve is used for interpolating the amount of profile error from the discrete deviation data.
The invention provides a method of manufacturing an object of measurement, comprising:
a deviation detection step of measuring a surface profile of an object of measurement through use of the surface profiling method as defined in any one of claims 5 through 9, thereby producing deviation data pertaining to a design equation of the object; and
a machining step of machining the profile of the object by means of feeding back the deviation data.
According to the invention, when the surface profile of an object of measurement is measured by means of causing a probe to follow a measurement surface of the object, the probe having a small-diameter probe of curvature radius of 1 mm or less provided at the tip end thereof, a profile error stemming from the curvature radius of the probe is determined through measurement of a reference ball to act as a reference for calibration. Measurement data pertaining to the object are corrected by use of stylus profile error data determined through measurement of the reference ball. At this time, a contact position between the object and the probe is determined, and the positional error stemming from the curvature radius of the probe is corrected on the basis of an angle of inclination of the measurement surface in the contact position. Further, the amount of profile error in the contact position is extracted from the stylus profile error data, and the profile error stemming from the curvature radius of the probe is corrected by addition or subtraction of the amount of profile error.
As a result, the profile error of the probe determined through measurement of the reference ball has been taken into consideration beforehand, and the error can be reflected in measurement data. Consequently, there can be provided accurate measurement data which have been appropriately calibrated in connection with the profile of the object. The surface profile of a small-size lens, such as an optical fiber condenser lens for use in the field of optical communication or a pickup lens for use with an optical disk, or the surface profile of a mold of the lens can be measured without the extremity of a probe interfering with an edge surface or lens barrel of the small-diameter lens. For example, the entire effective range of the lens can be measured with a high precision of 50 nm or less.